The present invention generally relates to electric arc discharge switches, and more particularly to a high-coulomb transfer switch useful for high current pulse power supplies. Such power supplies have particular application as power supplies for high power lasers used in welding and machining operations.
The most widely used high-coulomb transfer switches are trigatron spark gaps and thyratrons. Thyratrons, however, have lower limits on voltage hold-off and current capability; therefore, spark gaps are preferred for those applications that are beyond the limits of thyratrons. Even so, spark gaps can suffer severe damage due to hot-spot formations on the electrodes caused by the Z-pinch mechanism which constricts the current in an area of a few millimeters diameter. This phenomenon is discussed in an article by C. W. Kimblin entitled "Anode Phenomena in Vacuum and Atmospheric Pressure Arcs", published in IEEE Transactions on Plasma Science, PS-2 (1974), beginning at page 310. Briefly stated, the Z-pinch mechanism manifests itself from the ponderomotive force F=J.times.B, where J is the current density and B is the magnetic induction due to the current. Minor improvements of the spark gap geometry and the adoption of special materials have been ineffective in alleviating the effects of the Z-pinch mechanism.
One approach which has been taken to avoid hot-spot formations is disclosed in U.S. Pat. No. 3,471,733 issued to Joseph A. Rich. The device disclosed in this patent comprises a primary arc-electrode in the form of a re-entrant conductor so that arcing current flowing along the re-entrant path results in zero net magnetic field between the arc-electrodes. By effectively eliminating the magnetic field, arc currents do not bunch up and form hot-spots. Instead, the arc currents are diffuse and stationery between the electrodes. The current capacity of the basic device was increased in the improvement disclosed in U.S. Pat. No. 3,509,404 also issued to Joseph A. Rich. In the device disclosed in this patent, both the inner and outer electrode assemblies are re-entrant, and the outer electrode assembly incorporates a helical slotted current-carrying portion to cause arc rotation without exposure thereof to arcing current paths.
While the basic re-entrant geometry of the device disclosed in U.S. Pat. No. 3,471,733 does cause a substantial elimination of azimuthal magnetic field, it does not completely eliminate that magnetic field. The current carrying capacity was therefore limited by the interelectrode magnetic field present which causes current bunching or hot-spot formation. The rotating current produced by the device disclosed in U.S. Pat. No. 3,509,404 does help to alleviate this problem. However, both the basic device and the improved device are designed to be operated with a vacuum. High pressure operation is not possible due to current filament formation. Since a high pressure switch is usually inexpensive to construct, there are potentially a greater number of applications for high pressure switches than vacuum switches.